JPH103689A - Optical system for recording and/or reproducing optical information recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make possible recording or reproducing information on two kinds of optical information recording media with different thickness of transparent substrates by correcting on-axis color aberration occurring due to wavelength fluctuation of a laser light source in an optical system. SOLUTION: The light source, a divergent angle conversion lens, etc., are moved along an optical axis. Thus, spherical aberration occurring due to a difference between the thickness of the transparent substrates is canceled by the spherical aberration occurring by changing a divergent angle of incident light on an objective lens. Then, for answering to a sudden change of an oscillation wavelength of a semiconductor laser, the occurrence amount of the color aberration in the variable range of the wavelength is suppressed to an extent followed by a focusing servo as a whole optical system at the time of answering to the optical information recording medium of high density recording. In such a case, when the color aberration amount occurring when the wavelength of the laser light source is fluctuated by 10nm is defined Δfb(μm), |Δfb|<2.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for recording / reproducing an optical information recording medium used in an optical disk device or the like.
In particular, the present invention relates to a recording and / or reproducing optical system of an optical information recording medium that enables information recording and reproduction of information on two or more types of optical information recording media having different thicknesses of transparent substrates.

[0002]

2. Description of the Related Art A conventional optical system for recording / reproducing an optical information recording medium (the optical system for recording / reproducing in the present invention is a recording and / or reproducing system)
Alternatively, it includes a reproduction optical system, that is, a recording optical system, a reproduction optical system, and an optical system for both recording and reproduction. 2), as is well known, a light beam emitted from a light source such as a semiconductor laser or the like is imaged on an information recording surface through a transparent substrate having a predetermined thickness by an objective lens onto an information recording surface. FIG. 12 shows an example of a pickup device for an optical information recording medium constituted by such an optical system. In the figure, a light beam emitted from a light source 1 such as a semiconductor laser enters a collimator lens 3 through a beam splitter 2,
The light beam is converted into a parallel light beam, is limited to a predetermined light beam by the stop 5, and enters the objective lens 6. When a parallel light beam enters, the objective lens 6 forms an almost aberration-free light spot on the information recording surface 8 through a transparent substrate 7 having a predetermined thickness. The light beam modulated and reflected by the information pits on the information recording surface 8 returns to the beam splitter 2 via the objective lens 6 and the collimator lens 3, where it is separated from the optical path from the laser light source 1 and is sent to the photodetector 9. Incident. The photodetector 9 is a multi-divided PIN photodiode, outputs a current proportional to the temperature of the incident light beam from each element, and sends this current to a detection circuit (not shown), where an information signal and a focus signal are output. Based on the error signal and the track error signal, the objective lens 6 is controlled by a two-dimensional actuator composed of a magnetic circuit, a coil, and the like, and the light spot position is always adjusted on the information track.

In such a pickup device for an optical information recording medium, a large NA (for example, NA 0.6) is used in order to reduce the light spot condensed by the objective lens 6, so that the condensed light flux If the thickness of the transparent substrate is shifted from that of the transparent substrate, a large spherical aberration is generated. For example, N
A0.6, wavelength 6 of laser light emitted from laser light source
35 nm, thickness of transparent substrate 0.6 mm, substrate refractive index 1.
When the substrate thickness is changed with the objective lens optimized under the condition of 58, as shown in FIG. 13, every time the thickness of the substrate is shifted by 0.01, the aberration increases by about 0.01λrms. Therefore,
0.07λr if the thickness of the transparent substrate is shifted ± 0.07mm
ms, and reaches a Marechal limit value which is a standard for performing normal reading.

For this reason, in the conventional example shown in FIG. 12, when the thickness of the transparent substrate 7 is changed from 0.6 mm to 1.2 mm, the objective lens 6 corresponding to 0.6 mm is replaced with 1. The reproduction is performed by switching between the objective lens 11 and the aperture 10 corresponding to a thickness of 2 mm. As another measure when the thickness of the transparent substrate is changed from 0.6 mm to 1.2 mm, two pickup devices are provided for a substrate having a thickness of 0.6 mm and a substrate having a thickness of 1.2 mm. Is also conceivable.

On the other hand, in a semiconductor laser used in such an apparatus, the oscillation wavelength rapidly changes (shifts by several nm) due to a change in output power or a change in an external environment such as temperature.
For this reason, the focusing servo of the objective lens cannot follow and a recording error or a reproduction error occurs. In an optical disc reproducing device, a recorded signal contains an error correction code, so that even if the oscillation wavelength suddenly changes and a reproducing error occurs, a slight reproducing error does not affect the reproduced sound. However, in the case of an optical disk storage device, such a storage error and a reproduction error increase a bit error rate (BER) and reduce reliability.

[0006]

As described above, in order to enable a single optical information recording medium recording / reproducing apparatus to reproduce a plurality of types of optical discs having different transparent substrate thicknesses, for example, the disc substrate thickness is set. Two switchable objective lenses corresponding to 0.6 mm and 1.2 mm, and two optical pickup devices for disk substrate thicknesses of 0.6 mm and 1.2 mm However, the information pickup device and the optical disk device cannot be made compact and low in cost. Also, the recording / reproducing optical system used in an optical disk storage device for recording information on an optical disk must be designed so that the change in the focal position falls within a range that the servo can follow even if the oscillation wavelength of the semiconductor laser changes abruptly. No. In view of the above, the present invention enables recording and / or reproduction of optical discs having different substrate thicknesses with a single pickup device, is compatible with each other, and furthermore, the oscillation wavelength of the semiconductor laser changes rapidly. It is an object of the present invention to obtain a recording / reproducing optical system for an optical information recording medium having a simple structure and a compact structure, in which chromatic aberration is corrected so that a focusing servo can follow, and good recording / reproduction can be performed.

[0007]

The recording and / or reproducing optical system for an optical information recording medium of the present invention is capable of recording or reproducing information on at least two types of optical information recording media having different thicknesses of transparent substrates. A first arrangement for condensing a light beam from the laser light source on the information recording surface via the transparent substrate of the first optical information recording medium, and a thickness larger than that of the first optical information recording medium. Each element such as a light source and a lens constituting an optical system is illuminated between a second optical information recording medium having a transparent substrate and a second arrangement for condensing light on the information recording surface through the transparent substrate. In the optical system which is movable in the axial direction, the optical system has a means for correcting axial chromatic aberration caused by a wavelength variation of the laser light source. In the first arrangement, f is a focal length of the objective lens. The wavelength of the laser light source fluctuates by 10 nm The amount of chromatic aberration that occurs when △ fb
(Μm), | △ fb | <2.0 (1)

The means for correcting the chromatic aberration can be that the local dispersion value νOL of the material constituting the objective lens satisfies the following condition. νOL> 1500 (2) where νOL is the local dispersion value of the material forming the objective lens, and the refractive index at the reference wavelength λ of the light source is nλ, the refractive index at the wavelength λ + 10 nm, and the refractive index at the wavelength λ−10 nm is nλ. _1, is represented by = NyuOL when the _{nλ 2 (nλ-1) /} (nλ 2 -nλ 1).

Further, the optical system includes a divergence angle changing lens for changing a divergence angle of divergent light from a laser light source to a small value, and the means for correcting chromatic aberration includes a divergence angle changing lens having at least one positive lens. And at least one negative lens are bonded to each other, and the lens has overcorrected chromatic aberration near the oscillation wavelength of the laser light source. Further, the means for correcting the chromatic aberration includes a chromatic aberration correcting lens for correcting the axial chromatic aberration caused by the wavelength variation of the laser light source in the optical path between the laser light source and the objective lens. The chromatic aberration correcting lens can be a cemented lens obtained by bonding one positive lens and one negative lens. At this time, the chromatic aberration correcting lens moves along the optical axis alone or integrally with the light source and / or the divergence angle conversion lens according to the first arrangement and the second arrangement. Can be.

[0010]

The recording / reproducing optical system corresponding to the first optical information recording medium having a small thickness of the transparent substrate, that is, a large recording density, is optimized for the optical information recording medium. In order to correspond to another optical information recording medium with a low recording density, the light source, the divergence angle conversion lens, etc. are moved along the optical axis, so that the spherical aberration generated due to the difference in the thickness of the transparent substrate is applied to the objective lens. It is possible to adopt a method of canceling out by the spherical aberration generated by changing the divergence angle of the incident light. In such an optical system, in order to cope with a rapid change in the oscillation wavelength of the semiconductor laser, when the first optical information recording medium of the high density recording is supported,
It is sufficient that the amount of chromatic aberration generated in the wavelength change range of the entire optical system is suppressed to an extent that the focusing servo can follow. This is because the influence of the residual chromatic aberration is relatively small when supporting the second optical information recording medium having a low recording density. Condition (1) indicates the allowable range of chromatic aberration when the first optical information recording medium is supported.

The first means for suppressing the occurrence of chromatic aberration is to prevent the occurrence of chromatic aberration by selecting a lens, particularly a glass material of an objective lens having a large refractive power. Suppression of chromatic aberration by this method can be achieved not only by a finite conjugate type optical system in which divergent light from a laser light source is directly focused on a recording / reproducing surface by an objective lens, but also by divergence angle conversion to reduce the divergence angle of divergent light from the laser light source. It is extremely effective also in an optical system provided with a lens (including a collimator lens). Condition (2) is a condition required for a glass material for this purpose.
Specifically, it is satisfied by a material such as fluorite.

In order to suppress the chromatic aberration generated in the entire optical system, a method of canceling the chromatic aberration generated in the objective lens by the chromatic aberration generated in another optical element can be adopted.
When the recording / reproducing optical system includes a divergence angle conversion lens, this divergence angle conversion lens is a positive / negative or a positive / negative / positive cemented lens, and has an overcorrected chromatic aberration near the wavelength used. Can cancel the chromatic aberration generated by the above. Further, a chromatic aberration correcting lens having almost no refracting power and having overcorrected chromatic aberration may be provided in the optical path between the light source or the divergence angle conversion lens and the objective lens. In order to simplify the configuration of the chromatic aberration correcting lens, it is desirable that the chromatic aberration correcting lens is a cemented lens obtained by bonding one positive lens and one negative lens. Further, it goes without saying that various means conventionally used, such as a diffraction lens, a hologram lens, and a heterogeneous lens, are also used as such chromatic aberration correcting means.

In order to change the correspondence between the first optical information recording medium and the second optical information recording medium, a light source, a divergence angle conversion lens, or a chromatic aberration correction lens is moved along the optical axis. The divergence angle conversion lenses may be individually moved, or appropriate elements such as a light source and a divergence angle conversion lens or a divergence angle conversion lens and a chromatic aberration correction lens may be integrally moved. As a method of correcting the chromatic aberration of the entire optical system, the selection of the glass material of the objective lens, the use of a divergence angle conversion lens having overcorrected chromatic aberration, and the use of a chromatic aberration correction lens can be used independently. It goes without saying that these methods may be used in combination.

[0014]

Embodiments of the optical system of the present invention will be described below. In the table, ri is the radius of curvature of the i-th surface from the light source side,
di is the thickness or interval on the optical axis between the i-th surface and the (i + 1) -th surface from the light source side in the first arrangement (DVD compatible), and di 'is the light source in the second arrangement (CD compatible). I-th from the side
The thickness or interval on the optical axis between the i-th surface and the (i + 1) -th surface, ni is the refractive index at the used wavelength of the medium between the i-th surface and the (i + 1) -th surface from the light source side, and νi is The local dispersion value of the medium between the i-th surface and the (i + 1) -th surface from the light source side is shown. The aspherical shape of the lens surface is represented by κ in an orthogonal coordinate system with the vertex of the surface as the origin and the optical axis direction as the X axis.
Is a conical number, Ai is an aspheric coefficient, and Pi (4 ≦ Pi) is a power of an aspheric surface.

(Equation 1) It is represented by

Embodiment 1 This embodiment suppresses the occurrence of chromatic aberration by selecting the material of the objective lens. The change between the first arrangement and the second arrangement is performed by moving the divergence angle conversion lens along the optical axis. The optical path diagram is shown in FIG.
FIGS. 2 and 3 show the spherical aberration and the chromatic aberration.

Table 1 Surface number rid di di 'ni ν i 0 Light source 1.00 1.00 1 ∞ 0.95 0.95 1.5 455 756.7 2 ２０ 20.41 5.00 3 72.876 1.701 .70 1.53000 662.5 4-14.500 31.59 45.00 5 1.880 2.60 2.60 1.443084 1958.44 -3.677 1.58 1.407 1.70 60 1.20 1.58000 386.7 Aspherical surface data Fourth surface κ = −9.223640 × 10 ⁻¹ Fifth surface κ = −4.995770 × 10 ⁻¹ A ₁ = −1.24600 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −2.04650 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = 4.2250 × 10 ⁻⁵ P ₃ = 8.00000 A ₄ = −2.839990 × 10 ⁻⁵ P ₄ = 1.0000 Sixth surface κ = −3.811190 A ₁ = 2.999380 × 10 ⁻² P ₁ = 4.0000 A ₂ = −9.994490 × 10 ⁻³ P ₂ = 6.0000 A ₃ = 1.860460 × 10 ⁻³ P ₃ = 8.0000 A ₄ = −1.44890 × 10 ⁻⁴ P ₄ = 10.0000 | △ fb in the first arrangement | = 0.9 μm

Embodiment 2 In this embodiment, the configuration of the divergence angle conversion lens is a cemented lens in which one positive lens and one negative lens are bonded together. The divergence angle changing lens is moved to the light source side along the optical axis of the optical system. The spherical aberration and chromatic aberration are shown in FIGS.

Table 2 Surface number rid di di 'ni ν i 0 Light source 19.794 12.394 1 61.466 3.26 3.26 1.882525 306.3 2 12.104 3.28 3.28 1.69404 661 0.03-16.237 5.00 12.40 4 2.039 2.60 2.60 1.49810 711.65 -5.387 1.56 1.376 ∞ 0.60 1.20 1.20 58000 386.7 Aspherical surface data Fourth surface κ = -4.13830 × 10 ⁻¹ A ₁ = −1.4430 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −2.446290 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = −2.74020 × 10 ⁻⁵ P ₃ = 8. 0000 A ₄ = −9.04900 × 10 ⁻⁶ P ₄ = 10.00000 Fifth surface κ = −2.25790 × 10 A ₁ = 9.89000 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −3 .43850 × 10 ^-3 P ₂ = 6.0000 A ₃ = 8.07870 × 10 ^-4 P ₃ = 8.0000 A ₄ = −6.887940 × 10 ^-5 P ₄ = 10.00000 In the first arrangement | △ fb | = 1.9 μm

Embodiment 3 In this embodiment, the divergence angle conversion lens is a cemented lens in which three positive, negative, and positive lenses are bonded together. Same as Example 2. 6 and 7 show the spherical aberration and the chromatic aberration.

Table 3 Surface number rid di di 'ni ν i 0 325.9 3 10.464 3.30 3.30 1.71009 645.5 4 -16.799 5.00 12.40 5 2.039 2.60 2.60 1.49810 711.6 6 -5. 387 1.56 1.377 7∞0.60 1.20 1.58000 386.7 Aspherical surface data Fifth surface κ = -4.13830 × 10 ⁻¹ A ₁ = −1.4430 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −2.446290 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = −2.74020 × 10 ⁻⁵ P ₃ = 8. 0000 A ₄ = −9.04900 × 10 ⁻⁶ P ₄ = 10.00000 Sixth surface κ = −2.25790 × 10 A ₁ = 9.89000 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −3 .43850 × 10 ^-3 P ₂ = 6.0000 A ₃ = 8.07870 × 10 ^-4 P ₃ = 8.0000 A ₄ = −6.887940 × 10 ^-5 P ₄ = 10.00000 In the first arrangement | △ fb | = 1.2 μm

Embodiment 4 In this embodiment, a chromatic aberration correcting lens having a small refractive power is used, and the objective lens is the same as in Embodiment 2.
In the change from the first arrangement to the second arrangement, only the divergence angle changing lens is moved toward the light source along the optical axis. 8 and 9 show the spherical aberration and the chromatic aberration.

Table 4 Surface number rid didi 'ni νi 0 Light source 22.04 11.54 1 72.876 1.70 1.70 1.53000 662.5 2 -14.500 5.00 15.50 3-8 .544 1.11 1.11 1.79444 351.8 4 4.843 1.51 1.51 1.75184 631.8 5-9.673 5.00 5.00 6 2.039 2.60 2.60 60 1.49810 711.6 7 −5.387 1.55 1.36.8 ０ 0.60 1.20 1.58000 386.7 Aspherical data Second surface κ = −9.223640 × 10 ⁻¹ Sixth surface κ = -4.13830 × 10 ⁻¹ A ₁ = −1.4430 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −2.446290 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = −2.74020 × 10 ⁻⁵ P ₃ = 8. 0000 A ₄ = −9.04900 × 10 ⁻⁶ P ₄ = 10.00000 7th surface κ = −2.25790 × 10 A ₁ = 9.89000 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −3 .43850 × 10 ^-3 P ₂ = 6.0000 A ₃ = 8.07870 × 10 ^-4 P ₃ = 8.0000 A ₄ = −6.887940 × 10 ^-5 P ₄ = 10.00000 In the first arrangement | △ fb | = 0.9 μm

[0019]

The effects of the optical system according to the present invention will be described in comparison with the conventional example. The comparative example is an example in which a divergence angle changing lens almost similar to that of the embodiment is moved in the optical system of the conventional example similar to the first embodiment, and its spherical aberration and chromatic aberration are shown in FIGS. Shown in Comparative example

Table 5: Surface number rid di di 'ni ν i 0 Light source 1.00 1.00 1 0.95 0.95 1.5455 756.7 2 20.41 5.00 3 72.876 1.70 1.70 1.70 1.53000 662.5 4-14.500 35.59 51.00 5 2.039 2.60 2.60 1.49810 711.6 6-5.387 1.55 1.36 7 0.6 1.6. 20 1.58000 386.7 Aspherical surface data Fourth surface κ = -9.2640 × 10 ^-1 Fifth surface κ = -4.13830 × 10 ⁻¹ A ₁ = −1.4430 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −2.446290 × 10 ⁻⁴ P ₂ = 6.0000 A ₃ = −2.74020 × 10 ⁻⁵ P ₃ = 8. 0000 A ₄ = −9.04900 × 10 ⁻⁶ P ₄ = 10.00000 Sixth surface κ = −2.25790 × 10 A ₁ = 9.89000 × 10 ⁻³ P ₁ = 4.0000 A ₂ = −3 .43850 × 10 ^-3 P ₂ = 6.0000 A ₃ = 8.07870 × 10 ^-4 P ₃ = 8.0000 A ₄ = −6.887940 × 10 ^-5 P ₄ = 10.00000 In the first arrangement | △ fb | = 2.4 μm As is clear from the above, in the comparative example, the amount of chromatic aberration | △ fb | generated by the wavelength variation of the laser light source is large, and is clear from the aberration diagram in the first arrangement. ,
In addition, it is clear that the chromatic aberration in the second arrangement is also reduced as a result, and the recording of the optical information recording medium capable of performing good recording / reproduction even with a rapid change in the oscillation wavelength of the semiconductor laser. And / or a reproduction optical system could be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an optical path of an optical system according to a first embodiment of the present invention. FIG. 1A is a diagram corresponding to a first optical information recording medium, and FIG.
Indicates a time corresponding to the second optical information recording medium.

FIG. 2 is a diagram showing spherical aberration and chromatic aberration of the optical system when the optical system is compatible with a first optical information recording medium according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating spherical aberration and chromatic aberration of the optical system when a second optical information recording medium is supported according to the first embodiment of the present invention.

FIG. 4 is a diagram illustrating spherical aberration and chromatic aberration of the optical system when the first optical information recording medium is supported according to the second embodiment of the present invention.

FIG. 5 is a diagram of spherical aberration and chromatic aberration of an optical system when a second optical information recording medium is supported according to the second embodiment of the present invention.

FIG. 6 is a diagram illustrating spherical aberration and chromatic aberration of an optical system when a first optical information recording medium is supported according to a third embodiment of the present invention.

FIG. 7 is a diagram showing spherical aberration and chromatic aberration of the optical system when a second optical information recording medium is supported according to the third embodiment of the present invention.

FIG. 8 is a diagram showing spherical aberration and chromatic aberration of the optical system when supporting the first optical information recording medium according to the fourth embodiment of the present invention.

FIG. 9 is a diagram of spherical aberration and chromatic aberration of the optical system when a second optical information recording medium is supported according to the fourth embodiment of the present invention.

FIG. 10 is a diagram of spherical aberration and chromatic aberration of the optical system of the comparative example corresponding to the first optical information recording medium.

FIG. 11 is a diagram of spherical aberration and chromatic aberration of the optical system of the comparative example corresponding to the second optical information recording medium.

FIG. 12 is an optical layout diagram showing one example of an optical system of a conventional optical information recording medium pickup.

FIG. 13 is a graph showing a change in wavefront aberration due to a change in thickness of a transparent substrate in a pickup optical system of an optical information recording medium.

[Explanation of symbols]

1 light source 2 beam splitter 3
Collimator lens 5,10 Aperture 6,11 Objective lens 7
Transparent substrate 8 Information recording surface 9 Photodetector

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshi Kibayashi Konica Co., Ltd. 1 Sakuracho, Hino-shi, Tokyo

Claims (6)

[Claims] 1. A light beam from a laser light source is transmitted to a first optical information recording medium by an objective lens so as to enable information recording or information reproduction on at least two types of optical information recording media having different thicknesses of transparent substrates. 1st arrangement | positioning for condensing on an information recording surface via the transparent substrate of 1st, and information recording via the transparent substrate of the 2nd optical information recording medium which has a transparent substrate thicker than a 1st optical information recording medium. Second for focusing on the surface
An optical system in which at least one element constituting the optical system is movable in the optical axis direction between the arrangement and the optical system, wherein the optical system corrects axial chromatic aberration caused by a wavelength variation of the laser light source. In the first arrangement, the wavelength of the laser light source is 10 nm.
The optical system for recording and / or reproducing information on an optical information recording medium is characterized by: | △ fb | <2.0, where 色 fb (μm) is the amount of chromatic aberration generated when the optical system is changed. 2. A lens that converts a divergence angle of divergent light from a laser light source into a small size so as to enable information recording or information reproduction on at least two types of optical information recording media having different thicknesses of a transparent substrate. A first arrangement for condensing a light beam from the lens on the information recording surface via the transparent substrate of the first optical information recording medium by the objective lens, and a transparent substrate thicker than the first optical information recording medium; An optical system in which the divergence angle conversion lens is movable in the optical axis direction between a second arrangement for condensing light on an information recording surface via a transparent substrate of a second optical information recording medium having Wherein the optical system has means for correcting axial chromatic aberration caused by a wavelength variation of the laser light source. In the first arrangement, f is a focal length of the objective lens,
When the amount of chromatic aberration generated when the wavelength of the laser light source fluctuates by 10 nm is △ fb (μm), | △ fb | <2.0 for recording and / or reproducing on an optical information recording medium. Optical system 3. The optical information recording apparatus according to claim 1, wherein said means for correcting chromatic aberration is such that a local dispersion value νOL of a material forming the objective lens satisfies the following condition. Optical system for recording and / or reproduction of a medium νOL> 1500 where νOL is the local dispersion value of the material constituting the objective lens, and the refractive index at the reference wavelength λ of the light source is nλ, the wavelength λ + 10 nm, and the refraction at the wavelength λ−10 nm. When the rates are nλ ₁ and nλ ₂ , respectively, it is represented by νOL = (nλ−1) / (nλ ₂ −nλ ₁ ). 4. The optical system includes a divergence angle changing lens for changing a divergence angle of divergent light from a laser light source to a small angle, and the means for correcting chromatic aberration includes a divergence angle changing lens having at least one positive lens. 4. The light according to claim 1, wherein the cemented lens is a cemented lens obtained by laminating at least one negative lens and has overcorrected chromatic aberration near the oscillation wavelength of the laser light source. Optical system for recording and / or reproduction of information recording medium 5. The chromatic aberration correcting means includes a chromatic aberration correcting lens for correcting axial chromatic aberration caused by a wavelength fluctuation of the laser light source in the optical path between the laser light source and the objective lens. And the chromatic aberration correcting lens is 1
5. The optical system for recording and / or reproducing an optical information recording medium according to claim 1, wherein the optical system is a cemented lens obtained by bonding one positive lens and one negative lens. 6. The chromatic aberration correcting lens moves along an optical axis integrally with a light source and / or a divergence angle conversion lens according to the first arrangement and the second arrangement. 6. The recording and / or reproducing optical system for an optical information recording medium according to claim 5, wherein the optical system is at a fixed position regardless of the arrangement and the second arrangement.